Catalytic Article for Treating Exhaust Gas

ABSTRACT

Provided is a catalytic article comprising (a) a flow through honeycomb substrate having channel walls; (b) a first NH 3 -SCR catalyst composition coated on and/or within the channel walls in a first zone; and (c) a second NH 3 -SCR catalyst composition coated on and/or within the channel walls in a second zone, provided that the first zone is upstream of the second zone and the first and second zones are adjacent or at least partially overlap; and wherein the first NH 3 -SCR catalyst comprises a first copper loaded molecular sieve having a copper to aluminum atomic ratio of about 0.1 to 0.375 and the second NH 3 -SCR catalyst comprises a second copper loaded molecular sieve having a copper-to-aluminum atomic ratio of about 0.3 to about 0.6.

BACKGROUND

1. Field of Invention

The present invention relates to catalytic articles and methods fortreating combustion exhaust gas.

2. Description of Related Art

Combustion of hydrocarbon-based fuel in engines produces exhaust gasthat contains, in large part, relatively benign nitrogen (N₂), watervapor (H₂O), and carbon dioxide (CO₂). But the exhaust gases alsocontain, in relatively small part, noxious and/or toxic substances, suchas carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC)from un-burnt fuel, nitrogen oxides (NO_(x)) from excessive combustiontemperatures, and particulate matter (mostly soot). To mitigate theenvironmental impact of flue and exhaust gas released into theatmosphere, it is desirable to eliminate or reduce the amount of theundesirable components, preferably by a process that, in turn, does notgenerate other noxious or toxic substances.

Typically, exhaust gases from lean burn gas engines have a net oxidizingeffect due to the high proportion of oxygen that is provided to ensureadequate combustion of the hydrocarbon fuel. In such gases, one of themost burdensome components to remove is NO_(x), which includes nitricoxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O). Thereduction of NO_(x) to N₂ is particularly problematic because theexhaust gas contains enough oxygen to favor oxidative reactions insteadof reduction. Notwithstanding, NO_(x) can be reduced by a processcommonly known as Selective Catalytic Reduction (SCR). An SCR processinvolves the conversion of NO_(x) in the presence of a catalyst and withthe aid of a reducing agent, such as ammonia, into elemental nitrogen(N₂) and water. In an SCR process, a gaseous reductant such as ammoniais added to an exhaust gas stream prior to contacting the exhaust gaswith the SCR catalyst. The reductant is absorbed onto the catalyst andthe NO_(x) reduction reaction takes place as the gases pass through orover the catalyzed substrate. The chemical equation for stoichiometricSCR reactions using ammonia is:

4NO+4NH₃+O₂→4N₂+6H₂O

2NO₂+4NH₃+O₂→3N₂+6H₂O

NO+NO₂+2NH₃→2N₂+3H₂O

Zeolites having an exchanged transition metal are known to be useful asSCR catalysts. Conventional small pore zeolites exchanged with copperare particularly useful in achieving high NO_(x) conversion at lowtemperatures. However, the interaction of NH₃ with NO absorbed onto thetransition metal of an exchanged zeolite can lead to an undesirable sidereaction that produces N₂O. This N₂O is particularly problematic toremove from the exhaust stream. Accordingly, there remains a need forimproved methods that result in a high conversion of NO_(x) with minimalN₂O production. The present invention satisfies this need amongstothers.

SUMMARY OF THE INVENTION

Applicants have found that a catalytic substrate having at least twocatalytic zones, each of which contains a copper loaded molecular sieve,can substantially reduce the undesirable production of N₂O whilemaintaining overall high N₂ selectivity in an SCR reaction, providedthat the copper loaded molecular sieve catalyst positioned in anupstream zone has a lower Cu:Al molar ratio compared to the copperloaded molecular sieve in a downstream zone. For example, high N₂selectivity and low N₂O byproduct can be achieved in an SCR process byan exhaust gas through or past substrate having an upstream NH₃-SCRcatalyst zone comprising a copper loaded molecular sieve having a Cu:Almolar ratio of about 0.1 to 0.375 and a downstream NH₃-SCR catalyst zonehaving a Cu:Al molar ratio of about 0.3 to 0.6. In a preferredembodiment, the upstream molecular sieve has a higher silica-to-alumina(SAR) ratio relative to the downstream molecular sieve. In anotherpreferred embodiment, the upstream molecular sieve catalyst has a lowercopper loading relative to the downstream molecular sieve catalyst. Inyet another preferred embodiment, the upstream molecular sieve catalysthas a higher SAR and a lower copper loading relative to the downstreammolecular sieve catalyst.

Accordingly, in one aspect provided is a catalytic article comprising(a) a flow through honeycomb substrate having an inlet side, an outletside, an axial length from the inlet side to the outlet side, and aplurality of channels defined by channel walls extending from the inletside to the outlet side; (b) a first NH₃-SCR catalyst composition coatedon and/or within the channel walls in a first zone; and (c) a secondNH₃-SCR catalyst composition coated on and/or within the channel wallsin a second zone, provided that the first zone is upstream of the secondzone and the first and second zones are in series, and further providedthat the first NH₃-SCR catalyst comprises a first copper loadedmolecular sieve having a copper to aluminum molar ratio of about 0.1 to0.375 and the second NH₃-SCR catalyst comprises a second copper loadedmolecular sieve having a copper-to-aluminum molar ratio of about 0.3 toabout 0.6.

In another aspect, provided is a catalyst article comprising (a) a flowthrough honeycomb substrate having an inlet side, an outlet side, anaxial length from the inlet side to the outlet side, and a plurality ofchannels defined by channel walls extending from the inlet side to theoutlet side; (b) a first catalytic zone consisting of a first washcoat;(c) a second catalytic zone consisting of the first washcoat and asecond washcoat; (d) a third catalytic zone consisting of the secondwashcoat; and (e) a fourth catalytic zone consisting of the secondwashcoat over a third washcoat; wherein the first washcoat contains afirst copper loaded molecular sieve, the second washcoat contains asecond copper loaded molecular sieve wherein the first and secondmolecular sieves are different materials, and the third washcoatcontains an ammonia oxidation catalyst, and wherein the first, second,third, and fourth zones are arranged in series on the substrate, eachzone is adjacent to the next zone in the series, the first zone isproximal to the inlet side, and the fourth zone is proximal to theoutlet side.

In yet another aspect of the invention, provided is a system fortreating an exhaust gas comprising (a) a catalytic article accordingdescribed herein; and (b) one or more exhaust gas treatment componentsselected from DOC, NAC, external NH₃ injector, secondary SCR catalyst,ASC, and particulate filter, wherein the catalytic article according toclaim 1 and the one or more exhaust gas treatment components are influid communication and are in series.

In another aspect of the invention, provided is a method for treating anexhaust gas comprising (a) contacting an exhaust gas comprising NO_(x)and NH₃ with a catalytic article according to claim 1; and (b)selectively reducing at least a portion of the NO_(x) to N₂ and H₂O.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram showing an embodiment of the invention with anarrangement of zoned SCR catalysts;

FIG. 2 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts;

FIG. 3 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts;

FIG. 4 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts;

FIG. 4A is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts;

FIG. 5 is a diagram showing an embodiment of the invention witharrangement of zoned SCR catalysts and an ammonia oxidation catalyst;

FIG. 6 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts;

FIG. 7 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts comprising two substrates;

FIG. 7A is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts comprising two substrates and an ASCzone;

FIG. 8 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts, wherein one of the zones is in anextruded catalyst body;

FIG. 8A is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts, wherein one of the zones is in anextruded catalyst body;

FIG. 8B is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts, wherein the zones are on an extrudedcatalyst body;

FIG. 9 is a diagram showing an embodiment of the invention with anotherarrangement of zoned SCR catalysts, wherein one of the zones is in anextruded catalyst body;

FIG. 10 is a diagram of a flow-through honeycomb substrate comprisingzoned SCR catalysts;

FIG. 10A is a diagram of a cell of a flow-through honeycomb substrate;and

FIG. 11 is a diagram of a system for treating exhaust gas according toan embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention, at least in part, is directed to a method for improvingenvironmental air quality, and in particular for treating exhaust gasemissions generated by power plants, gas turbines, lean burn internalcombustion engines, and the like. Exhaust gas emissions are improved, atleast in part, by reducing NO_(x) concentrations over a broadoperational temperature range. The conversion of NO_(x) is accomplishedpassing the exhaust gas through or over a substrate, preferably ahoneycomb flow-through monolith, having two or more NH₃-SCR catalystsarranged in zones.

Preferably, a first NH₃-SCR catalyst composition is coated on and/orwithin the channel walls of the flow-through monolith in a first zone,and a second NH₃-SCR catalyst composition is coated on and/or within thechannel walls of the flow-through monolith in a second zone, the firstzone being upstream of the second zone. In certain embodiments, thefirst or second zone can be in the form of an extruded catalyst body andthe other zone is a coating on the body. In certain embodiments, thefirst zone and the second zone are the same catalyst formulation,provided that the first zone has a lower washcoat loading compared tothe second zone. In one example, the first zone can have a washcoatloading that is 85%, 75%, 65%, or 50% of the rear washcoat loading. Incertain embodiments, the first and second zones having the same catalystformulation but different washcoat loadings are separated on differentsubstrates. Preferably, these two substrates are adjacent to oneanother. In certain embodiments, these substrates are disposed in anexhaust gas treatment system such that no other SCR catalyst, andpreferably no other exhaust gas treatment catalyst are positionedbetween the two substrates.

Turning to FIG. 1, shown is an embodiment of the invention in which aflow-through honeycomb substrate 10 has a first catalyst zone 20 and asecond catalyst zone 30, wherein the first and second catalyst zones areconsecutive and in contact. The terms “first zone” and “second zone” areindicative of the zone's orientation on the substrate. Morespecifically, the zones are oriented in series so that under normaloperating conditions, the exhaust gas to be treated contacts the firstzone prior to contacting the second zone. In one embodiment, the firstand second zones are arranged consecutively, so that one follows theother in an uninterrupted succession (i.e., there is no catalyst orother exhaust gas treatment operation such as a filter between the firstand second zones). Therefore, in certain embodiments, the first zone isupstream of the second zone with respect to normal exhaust gas flow 1through or over the substrate.

Differences in the catalyst materials of the first and second zonesresult in different treatment of the exhaust gas as it passes through orover the substrate. For example, the first zone selectively reducesNO_(x) while generating lower N₂O byproduct relative to the second zone,and the second zone selectively reduces NO_(x) at a higher selectivityrelative to the first zone. The synergistic effect of the combination ofthe two zones improves the overall performance of the catalyst comparedto single catalyst systems or other zoned arrangements.

In FIG. 10, a zoned catalytic substrate 2 is shown wherein the substrateis a honeycomb flow-through monolith 100 having an inlet side 110 and anoutlet side 120, relative to the normal direction of exhaust gas flow 1through the substrate. The substrate has an axial length 190 thatextends from the inlet side 110 to the outlet side 120. FIG. 11 shows asingle cell 200 of a honeycomb substrate having channel walls 110 thatdefine open channels 120 through which exhaust gas can flow. The channelwalls are preferable porous or semi-porous. The catalyst of each zonecan be a coating on the surface of the walls, a coating that permeatespartially or fully into the walls, incorporated directly into the wallas an extruded body, or some combination thereof.

In FIG. 1, the first zone 20 extends from the inlet side 110 to a firstend point 29 that is positioned about 10 to 90 percent, for exampleabout 80-90 percent, about 10 to 25 percent or about 20-30 percent, ofthe axial length 190. The second zone 120 extends from the outlet side120 about 20 to 90 percent, for example about 60 to about 80 percent orabout 50 to about 75 percent, of the axial length 190. Preferably, thesecond zone extends to at least the first end point so that the firstand second zones are in contact. The axial length is preferably lessthan 24 inches, such as about 1 to about 24 inches, about 3 to about 12inches, or about 3 to about 6 inches.

In FIG. 2, the first catalyst zone 20 partially overlaps the secondcatalyst zone 30. In FIG. 3, the second catalyst zone 30 partiallyoverlaps the first catalyst zone 20. The overlap is preferably less than90 percent of the axial length of the substrate, for example about 80 toabout 90 percent, less than about 40, about 40 to about 60, about 10 toabout 15 percent, or about 10 to about 25 percent. For embodiments inwhich the second catalyst zone overlaps the first catalyst zone, theoverlap can be greater than 50 percent of the axial length, such as80-90 percent. For embodiments in which the first catalyst zone overlapsthe second catalyst zone, the overlap is preferably less than 50 percentof the axial length, for example about 10-20 percent.

In FIG. 4, the first catalyst zone 20 completely overlaps the secondcatalyst zone 30. Preferably, the first and second zones are in contact(i.e., there are no intervening catalytically active layers between thefirst and second zones). For such embodiments, the exhaust gas to betreated first contacts and is at least partially treated by the firstzone. At least a portion of the exhaust gas permeates through the firstzone where it contacts the second zone where it is subsequently treated.A least a portion of the treated exhaust gas permeates back through thefirst zone, enters the open channel, and exits the substrate. FIG. 4shows an embodiment in which the first and second catalyst zones extendthe entire axial length of the substrate. FIG. 4A shows an embodiment inwhich the first catalyst zone completely overlaps the second zone, thefirst catalyst zone extends the entire axial length of the substrate,and the second zone extends from the outlet side to less than the fullaxial length of the substrate.

FIG. 5 shows another embodiment of the invention. Here, the catalyticarticle further comprises a third catalyst zone proximal to, andpreferably extending to, the outlet side of the substrate. The thirdcatalyst zone comprises an oxidation catalyst, preferably a catalysteffective to oxidize ammonia. In certain embodiments, the catalystcomprises one or more platinum group metals (PGM), such as Pt, Pd, or acombination thereof, preferably on a metal oxide support, such asalumina. The combination of the second and third zones in a layeredarrangement serves as an ammonia slip catalyst, wherein at least aportion of the excess ammonia not consumed by the upstream SCR reactionpasses through the second zone to the third zone where it is oxidizedinto H₂O and secondary NO_(x). The H₂O and secondary NO_(x) pass backthrough the second zone where at least a portion of the secondary NO_(x)is reduced to N₂ and H₂O via an SCR-type reaction.

Another embodiment of the invention is shown in FIG. 6 wherein thesubstrate supports four discrete catalyst zones wherein each comprises aseparate catalyst composition. The first, second, and third catalystzones are similar in composition to those described above. The forthcatalyst zone is positioned between the first and second zones so thatthe first, fourth, and second zones are in series, the first zonecontacts the fourth zone, and the fourth zone contacts the second zone.In certain embodiments, the fourth zone can have two or more catalystlayers, wherein each catalyst layer comprises a copper loaded molecularsieve.

The total amount of copper per linear inch in the fourth zone is greaterthan the total amount of copper per linear inch of the first zones andless than that of the second zone or is greater than the total amount ofcopper per linear inch of the first and second zones, individually. Incertain embodiments, the molecular sieve of the fourth zone is the sameas the molecular sieve in the first zone and/or second zone. In certainembodiments, the molecular sieve of the fourth zone comprises twomolecular sieve materials. For example, the fourth zone can comprise themolecular sieve of the first zone and the molecular sieve of the secondzone, provided that the first and second zones have different molecularsieves.

Preferably, the first and second zones are consecutively arranged on asingle substrate so that the first zone contacts the second zone. Incertain embodiments, the first and second zones are arranged on separatesubstrates which are arranged in an exhaust gas treatment system so thatthe first and second zones are in series and are in contact. The twosubstrates can be the same or different substrates. For example, thefirst substrate can have a higher porosity than the second substrate,the first and second substrates can be different compositions or have adifferent cell density, and/or the first and second substrates can bedifferent lengths. In FIG. 7, the first and second zones are arranged onseparate substrates which are arranged in an exhaust gas treatmentsystem so that the first and second zones are in series and areadjacent, but are not in direct contact. The maximum distance betweenthe first and second substrates is preferably less than 2 inches, morepreferably less than 1 inch, and preferably there are no interveningsubstrates, filters, or catalyst materials between the first and secondzones and/or between the first and second substrates. In FIG. 7A, thesecond substrate further comprises an ammonia oxidation catalystunder-layer 40 that extends from the outlet side of the substrate to alength less than the total length of substrate. The second zonecompletely covers the oxidative catalyst and preferably extends thelength of the substrate.

In certain embodiments, the first or second catalyst zone comprises anextruded catalyst material. The embodiment shown in FIG. 8, for example,comprises a first catalytic zone 26 in the form of a coating on and/orwithin a portion of an extruded catalyst substrate. The extrudedcatalyst substrate, in turn, comprises the second catalytic zone 16. Thefirst zone is arranged on the substrate so that it is upstream of thesecond zone with respect to the normal flow of exhaust gas 1. Thecatalytically active substrate in zone 16 comprises a catalyticallyactive material similar to that of the other second zones describedherein. In FIG. 8, the first zone extends from the inlet side to lessthan the full length of the substrate. In FIG. 84, the first zone 26completely covers the catalytically active substrate comprising thesecond zone.

In FIG. 8B, a catalytically active substrate 300, for example aflow-through honeycomb substrate formed from an extruded catalyticmaterial, is coated with an upstream zone 310 and a downstream zone 330.The upstream zone extends from the inlet side 312 to a first end point314 that is positioned about 10 to 80 percent, for example about 50-80percent, about 10 to 25 percent, or about 20-30 percent, of the axiallength 390. The downstream zone extends from the outlet side 344 to asecond end point 332 that is position about 20 to 80 percent, forexample about 20-40 percent, about 60 to about 80 percent, or about 50to about 75 percent, of the axial length 390. The upstream zone anddownstream zone are not in direct contact and thus a gap 320 existsbetween the upstream zone and the downstream zone. Preferably, this gapdoes not contain a catalyst layer but instead is directly exposed to theexhaust gas to be treated. The exhaust gas contacts the catalytic bodyat the gap whereby the exhaust gas is treated, for example toselectively reduce a portion of NO_(x) in the exhaust gas. The gap,which is defined by the first end point 314 and the second end point332, is preferably less than 75 percent of the axial length, for exampleabout 40 to about 60, about 10 to about 15 percent, or about 10 to about25 percent of the axial length 390. An optional NH₃ oxidation catalystis coated on and/or within the substrate 300 in a zone that extends fromthe outlet side 344 towards the inlet side 312 for a length that isequal to or less than the length of the downstream zone. The optionalNH₃ oxidation catalyst is preferably an under-layer that is completelycovered by the catalyst composition forming the downstream zone.

The compositions of the catalysts in the upstream zone, the extrudedbody, and the downstream zone are not particularly limited provided thatat least two of the upstream zone, the extruded body, and the downstreamzone conform to the first and second zone requirements as definedherein, that is the copper-loaded molecular sieve in the first zone hasa Cu:Al ratio that is less than the Cu:Al ratio of the copper-loadedmolecular sieve in the second zone. In one example, the upstream zonecorresponds to the first zone and the downstream zone corresponds to thesecond. In such embodiments, the extruded catalyst body preferablycomprises another type of SCR catalyst, such as vanadium, preferablysupported on a metal oxide such as TiO₂, and optionally comprising oneor more additional metals such as tungsten. In another example, theextruded catalyst body corresponds to the first zone and the downstreamzone corresponds to the second zone. In this example, the upstream zonecan comprising another type of catalyst, preferably an SCR catalyst suchas an iron loaded molecular sieve. In another example, the upstream zonecorresponds to the first zone and the extruded body corresponds to thesecond zone. In this example, this downstream zone can comprise anotherpreferably comprises another type of SCR catalyst such as one of thosedescribed herein.

FIG. 9 shows another embodiment wherein a first catalytic zone 17 ispart of an extruded catalytic body and the second catalyst zone 37 is acoating on and/or within a portion of the extruded catalyst substrate.Again, the first zone is arrange upstream of the second zone withrespect to the normal flow of exhaust gas 1 and the catalytically activesubstrate in zone 17 comprises a catalytically active material similarto that of the other first zones described herein.

The first catalytic zone comprises a first NH₃-SCR catalyst composition.The first NH₃-SCR catalyst comprises a copper-loaded molecular sieve asa catalytically active component, but may include other components,particularly catalytically inactive components such as binders. As usedherein, a “catalytically active” component is one that directlyparticipates in the catalytic reduction of NO_(x) and/or oxidization ofNH₃ or other nitrogenous-based SCR reductants. By corollary, a“catalytically inactive” component is one which does not directlyparticipate in the catalytic reduction of NO_(x) and/or oxidization ofNH₃ or other nitrogenous-based SCR reductants.

Useful molecular sieves are crystalline or quasi-crystalline materialswhich can be, for example, aluminosilicates (zeolites) orsilicoaluminophosphates (SAPOs). Such molecular sieves are constructedof repeating SiO₄, AlO₄, and optionally PO₄ tetrahedral units linkedtogether, for example in rings, to form frameworks having regularintra-crystalline cavities and channels of molecular dimensions. Thespecific arrangement of tetrahedral units (ring members) gives rise tothe molecular sieve's framework, and by convention, each uniqueframework is assigned a unique three-letter code (e.g., “CHA”) by theInternational Zeolite Association (IZA). Examples of useful molecularsieve frameworks include large pore frameworks (i.e., having a minimumring size of 12-members), medium pore frameworks (i.e., having a minimumring size of 10-members), and small-pore frameworks (i.e., having aminimum ring size of 8-members). Examples of frameworks include BEA,MFI, CHA, AEI, LEV, KFI, MER, RHO, ERI, OFF, FER, and AFX. The molecularsieve can also be an intergrowth of two or more frameworks, such as AEIand CHA. In certain embodiments, the first and/or second zones canindependently comprise a blend of two or more molecular sieves.Preferred blends have at least one molecular sieve having a CHAframework, and more preferably a majority of the CHA framework.

Particularly useful molecular sieve are small pore zeolites. As usedherein, the term “small pore zeolite” means a zeolite framework having amaximum ring size of eight tetrahedral atoms. Preferably, the primarycrystalline phase of the molecular sieve is constructed of one or moresmall pore frameworks, although other molecular sieve crystalline phasesmay also be present. Preferably, the primary crystalline phase comprisesat least about 90 weight percent, more preferably at least about 95weight percent, and even more preferably at least about 98 or at leastabout 99 weight percent small pore molecular sieve framework, based onthe total amount of molecular sieve material.

In some examples, the small pore zeolite for use in the presentinvention have a pore size in at least one dimension of less than 4.3 Å.In one embodiment, the small pore zeolite has a framework selected fromthe group of consisting of: ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD,ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW,LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO,TSC, UEI, UFI, VNI, YUG and ZON. Preferred zeolite frameworks areselected from AEI, AFT, AFX, CHA, DDR, ERI, LEV, KFI, RHO, and UEI. Forcertain applications, preferred zeolite frameworks are selected fromAEI, AFT, and AFX, particularly AEI. In certain application, a preferredzeolite framework is CHA. In certain applications, an ERI framework ispreferred. Particular zeolites that are useful for the present inventioninclude SSZ-39, Mu-10, SSZ-16, SSZ-13, Sigma-1, ZSM-34, NU-3, ZK-5, andMU-18. Other useful molecular sieves include SAPO-34 and SAPO-18. In aparticularly preferred embodiment, the first and second NH₃-SCRcatalysts independently comprise an aluminosilicate having a CHAframework (e.g., SSZ-13) loaded with copper. In another particularlypreferred embodiment, the first NH₃-SCR catalyst comprises acopper-loaded SAPO-34 molecular sieve and the second NH₃-SCR catalystcomprises a copper-loaded aluminosilicate having a CHA framework.

Preferred aluminosilicates have a silica-to-alumina ratio (SAR) of about10 to about 50, for example about 15 to about 30, about 10 to about 15,15 to about 20, about 20 to about 25, about 15 to about 18, or about 20to about 30. Preferred SAPOs have an SAR of less than 2, for exampleabout 0.1 to about 1.5 or about 0.5 to about 1.0. The SAR of a molecularsieve may be determined by conventional analysis. This ratio is meant torepresent, as closely as possible, the ratio in the rigid atomicframework of the molecular sieve crystal and to exclude silicon oraluminum in the binder or in cationic or other form within the channels.Since it may be difficult to directly measure the SAR of the molecularsieve after it has been combined with a binder material, particularly analumina binder, the SAR value described herein is expressed in terms ofthe SAR of the molecular sieve per se, i.e., prior to the combination ofthe zeolite with the other catalyst components.

In certain applications, the SAR of the molecular sieve in the firstzone is less than the SAR of the molecular sieve in the second zone. Forexample, the first zone molecular sieve can be an aluminosilicate havingan SAR of about 10 to about 20 and the second zone molecular sieve canbe an aluminosilicate having an SAR of about 20 to about 50. In anotherexample, the first zone molecular sieve can be an aluminosilicate havingan SAR of about 15 to about 20 and the second zone molecular sieve canbe an aluminosilicate having an SAR of about 25 to about 30. In anotherexample, the first zone molecular sieve is a SAPO and the second zonemolecular sieve is an aluminosilicate. In other embodiments, the firstzone molecular sieve and the second zone molecular sieve have the sameSAR, provided that the copper loading on the first zone molecular sieveis higher than the copper loading on the second zone molecular sieve.

The molecular sieve may include framework metals other than aluminum(i.e., metal-substituted zeolites). As used herein, the term “metalsubstituted” with respect to a molecular sieve means a molecular sieveframework in which one or more aluminum or silicon framework atoms hasbeen replaced by the substituting metal. In contrast, the term “metalexchanged” means a molecular sieve in which one or more ionic speciesassociated with the zeolite (e.g., H⁺, NH4⁺, Na⁺, etc.) has beenreplaced by a metal (e.g., a metal ion or free metal, such as metaloxide), wherein the metal is not incorporated as a molecular sieveframework atom (e.g., T-atom), but instead is incorporated into themolecular pores or on the external surface of the molecular sieveframework. The exchanged metal is a type of “extra-framework metal”,that is a metal that resides within the molecular sieve and/or on atleast a portion of the molecular sieve surface, preferably as an ionicspecies, does not include aluminum, and does not include atomsconstituting the framework of the molecular sieve. The terms“metal-loaded molecular sieve” means a molecular sieve that includes oneor more extra-framework metals. As used herein, the terms“aluminosilicate” and “silicoaluminophosphate” are exclusive ofmetal-substituted molecular sieves.

The copper-loaded molecular sieves of the present invention comprisecopper disposed on and/or within the molecular sieve material as anextra-framework metal. Preferably, the presence and concentration of thecopper facilitates the treatment of exhaust gases, such as exhaust gasfrom a diesel engine, including processes such as NO_(x) reduction, NH₃oxidation, and NO_(x) storage, while also suppressing the formation ofN₂O.

The copper (Cu) is present in an amount relative to the amount ofaluminum (Al) in the molecular sieve, namely the framework aluminum. TheCu:Al ratio is based on the relative molar amount of copper to molarframework Al in the molecular sieve. The copper-loaded molecular sievein the first zone has a copper-to-aluminum molar ratio (Cu:Al ratio)that is less than the Cu:Al ratio of the copper-loaded molecular sievein the second zone. Applicants have surprisingly found that adjustingthe Cu:Al ratio of molecular sieves between the upstream and downstreamzones of a flow-through honeycomb substrate provides high NO_(x)conversion (particularly NO and NO₂) and N₂ selectivity in an SCRprocess, while substantially reducing the amount of unwanted N₂Oconcurrently generated as a byproduct. A difference Cu:Al ratio in theupstream catalytic zone compared to the downstream catalytic zones canbe achieve by loading the molecular sieve in the upstream zone with lesscopper relative to the molecular sieve in the downstream zone, byincreasing the SAR of the molecular sieve in the downstream zonerelative to the molecular sieve in the upstream zone, or a combinationthereof.

Unless otherwise specified, the amount of copper loaded onto a molecularsieve and the copper concentration in the catalyst is referenced interms of the copper per total weight of the corresponding molecularsieve, and is thus independent of the amount of catalyst washcoatloading on the substrate or the presence of other materials in thecatalyst washcoat.

For certain applications, the Cu:Al ratio of the catalyst in the firstzone is about 0.1 to about 0.375 and the Cu:Al ratio of the catalyst inthe second zone is about 0.3 to about 0.6, provided that the Cu:Al ratioin the first zone catalyst is less than the Cu:Al ratio in the secondzone catalyst. In certain embodiment, the first zone comprises copperloaded SAPO and has a Cu:Al ratio of about 0.01 to about 0.1. In certainembodiments the first zone comprises an aluminosilicate and has a Cu:Alratio of about 0.15 to about 0.375. In certain embodiments, the firstzone comprises a SAPO and the second zone comprises an aluminosilicatemolecular sieve, wherein the SAPO and aluminosilicate molecular sieveare loaded with a comparable amount of copper. In another embodiment,the molecular sieves of the first and second zones are bothaluminosilicates having comparable SARs, provided that the molecularsieve of the first zone is loaded with a lower concentration of copperrelative to the molecular sieve of the second zone. In anotherembodiment, the molecular sieves of the first and second zones are bothaluminosilicates wherein the aluminosilicate of the first zone has alower SAR relative to the molecular sieve of the second zone and the twoaluminosilicates have the same copper loading or the first zonemolecular sieve has a lower copper loading relative to the second zonemolecular sieve.

In certain embodiments, the extra-framework copper is present in themolecular sieve of the first zone or second zone at a concentration ofabout 0.1 to about 10 weight percent (wt %) based on the total weight ofthe molecular sieve, for example from about 0.5 wt % to about 5 wt %,from about 0.5 to about 1 wt %, from about 1 to about 5 wt %, about 2.5wt % to about 3.5 wt %, and about 3 wt % to about 3.5 wt %.

In addition to copper, the molecular sieve can further comprise one ormore additional extra-framework metals, provided that the additionalextra-framework metal is present in a minority amount (i.e., <50 mol. %,such as about 1 to 30 mol. %, about 1-10 mol. % or about 1-5 mol. %)relative to the copper. The additional extra-framework metal may be anyof the recognized catalytically active metals that are used in thecatalyst industry to form metal-exchanged molecular sieves, particularlythose metals that are known to be catalytically active for treatingexhaust gases derived from a combustion process. Particularly preferredare metals useful in NO_(x) reduction and storage processes. Examples ofsuch metals include metal nickel, zinc, iron, tungsten, molybdenum,cobalt, titanium, zirconium, manganese, chromium, vanadium, niobium, aswell as tin, bismuth, and antimony; platinum group metals, such asruthenium, rhodium, palladium, indium, platinum, and precious metalssuch as gold and silver. Preferred transition metals are base metals,and preferred base metals include those selected from the groupconsisting of chromium, manganese, iron, cobalt, nickel, and mixturesthereof.

Preferably, the copper is highly dispersed within the molecular sievecrystals, preferably without a high temperature treatment of the metalloaded molecular sieve. For embodiments which utilize copper, the copperloading is preferably fully ion exchanges and/or is preferably less thancan be accommodated by the exchange sites of the molecular sievesupport. Preferably, the catalyst is free or substantially free of bulkcopper oxide, free or substantially free of species of copper onexternal molecular sieve crystal surfaces, and/or free or substantiallyfree of copper metal clusters as measured by temperature programmedreduction (TPR) analysis and/or UV-vis analysis.

In one example, a metal-exchanged molecular sieve is created by mixingthe molecular sieve, for example an H-form molecular sieve or anNH₄-form molecular sieve, into a solution containing soluble precursorsof the catalytically active metal(s). The pH of the solution may beadjusted to induce precipitation of the catalytically active metalcations onto or within the molecular sieve structure (but not includingthe molecular sieve framework. For example, in a preferred embodiment, amolecular sieve material is immersed in a solution containing coppernitrate or copper acetate for a time sufficient to allow incorporationof the catalytically active copper cations into the molecular sievestructure by ion exchange. Un-exchanged copper ions are precipitatedout. Depending on the application, a portion of the un-exchanged ionscan remain in the molecular sieve material as free copper. Themetal-exchanged molecular sieve may then be washed, dried, and calcined.The calcined material may include a certain percentage of copper ascopper oxide residing on the surface of the molecular sieve or withinthe molecular sieve cavities.

Generally, ion exchange of the catalytic metal cation into or on themolecular sieve may be carried out at room temperature or at atemperature up to about 80° C. over a period of about 1 to 24 hours at apH of about 7. The resulting catalytic molecular sieve material ispreferably dried at about 100 to 120° C. overnight and calcined at atemperature of at least about 500° C.

In certain embodiments, the catalyst composition comprises thecombination of copper and at least one alkali or alkaline earth metal,wherein the copper and alkali or alkaline earth metal(s) are disposed onor within the molecular sieve material. The alkali or alkaline earthmetal can be selected from sodium, potassium, rubidium, cesium,magnesium, calcium, strontium, barium, or some combination thereof. Asused here, the phrase “alkali or alkaline earth metal” does not mean thealkali metals and alkaline earth metals are used in the alternative, butinstead that one or more alkali metals can be used alone or incombination with one or more alkaline earth metals and that one or morealkaline earth metals can be used alone or in combination with one ormore alkali metals. In certain embodiments, alkali metals are preferred.In certain embodiments, alkaline earth metals are preferred. Preferredalkali or alkaline earth metals include calcium, potassium, andcombinations thereof.

In certain embodiments, the catalyst composition is essentially free ofmagnesium and/or barium. In certain embodiments, the catalyst isessentially free of any alkali or alkaline earth metal except calciumand potassium. In certain embodiments, the catalyst is essentially freeof any alkali or alkaline earth metal except calcium. And in certainother embodiments, the catalyst is essentially free of any alkali oralkaline earth metal except potassium. As used herein, the term“essentially free” means that the material does not have an appreciableamount of the particular metal. That is, the particular metal is notpresent in amount that would affect the basic physical and/or chemicalproperties of the material, particularly with respect to the material'scapacity to selectively reduce or store NO_(x). In certain embodiments,the molecular sieve material has an alkali content of less than 3 weightpercent, more preferably less than 1 weight percent, and even morepreferably less than 0.1 weight percent.

In certain embodiments, the alkali and/or alkaline earth metal(collectively A_(M)) is present in the molecular sieve material in anamount relative to the amount of copper in the molecular sieve.Preferably, the Cu and A_(M) are present, respectively, in a molar ratioof about 15:1 to about 1:1, for example about 10:1 to about 2:1, about10:1 to about 3:1, or about 6:1 to about 4:1, particularly were A_(M) iscalcium. In certain embodiments which include an alkali and/or alkalineearth metal such as calcium, the amount of copper present is less than2.5 weight percent, for example less than 2 weight percent or less than1 weight percent, based on the weight of the molecular sieve. In certainembodiments, the copper loaded molecular sieve of the second zonecontains an alkali or alkaline earth metal, particularly calcium, andthe copper loaded molecular sieve of the first zone is essentially freeof an alkali or alkaline earth metal. For such embodiments, the relativecumulative amount of copper and alkali and/or alkaline earth metal(A_(M)) present in the molecular sieve material of the second zone isrelative to the amount of aluminum in the molecular sieve, namely theframework aluminum. As used herein, the (Cu+A_(M)):Al ratio is based onthe relative molar amounts of Cu+A_(M) to molar framework Al in thecorresponding molecular sieve. In certain embodiments, the molecularsieve of the second zone has a (Cu+A_(M)):Al ratio of not more thanabout 0.6, particularly where A_(M) is calcium. In certain embodiments,the (Cu+A_(M)):Al ratio is at least 0.3, for example about 0.3 to about0.6. In such embodiments, the Cu:Al ratio of the catalyst in the firstzone is about 0.1 to about 0.375, provided that the Cu:Al ratio in thefirst zone catalyst is less than the (Cu+A_(M)):Al ratio in the secondzone catalyst.

In certain embodiments, the relative cumulative amount of copper andalkali and/or alkaline earth metal (A_(M)) is present in the molecularsieve material of the second zone in an amount relative to the amount ofaluminum in the molecular sieve, namely the framework aluminum. As usedherein, the (Cu+A_(M)):Al ratio is based on the relative molar amountsof Cu+A_(M) to molar framework Al in the corresponding molecular sieve.In certain embodiments, the catalyst material has a (Cu+A_(M)):Al ratioof not more than about 0.6. In certain embodiments, the (Cu+A_(M)):Alratio is not more than 0.5, for example about 0.05 to about 0.5, about0.1 to about 0.4, or about 0.1 to about 0.2.

The alkali/alkaline earth metal can be added to the molecular sieve viaany known technique such as ion exchange, impregnation, isomorphoussubstitution, etc. The copper and the alkali or alkaline earth metal canbe added to the molecular sieve material in any order (e.g., the metalcan be exchanged before, after, or concurrently with the alkali oralkaline earth metal), but preferably the alkali or alkaline earth metalis added prior to or concurrently with the copper.

The catalytic articles of the present invention are applicable forheterogeneous catalytic reaction systems (i.e., solid catalyst incontact with a gas reactant). To improve contact surface area,mechanical stability, and/or fluid flow characteristics, the SCRcatalysts are disposed on and/or within a substrate such as honeycombcordierite brick. In certain embodiments, one or more of the catalystcompositions is applied to the substrate as a washcoat(s).Alternatively, one or more of the catalyst compositions is kneaded alongwith other components such as fillers, binders, and reinforcing agents,into an extrudable paste which is then extruded through a die to form ahoneycomb brick.

Certain aspects of the invention provide a catalytic washcoat. Thewashcoat comprising a copper-loaded molecular sieve catalyst describedherein is preferably a solution, suspension, or slurry. Suitablecoatings include surface coatings, coatings that penetrate a portion ofthe substrate, coatings that permeate the substrate, or some combinationthereof.

A washcoat can also include non-catalytic components, such as fillers,binders, stabilizers, rheology modifiers, and other additives, includingone or more of alumina, silica, non-molecular sieve silica alumina,titania, zirconia, ceria. In certain embodiments, the catalystcomposition may comprise pore-forming agents such as graphite,cellulose, starch, polyacrylate, and polyethylene, and the like. Theseadditional components do not necessarily catalyze the desired reaction,but instead improve the catalytic material's effectiveness, for example,by increasing its operating temperature range, increasing contactsurface area of the catalyst, increasing adherence of the catalyst to asubstrate, etc. In preferred embodiments, the washcoat loading is >0.3g/in³, such as >1.2 g/in³, >1.5 g/in³, >1.7 g/in³ or >2.00 g/in³, andpreferably <3.5 g/in³, such as <2.5 g/in³. In certain embodiments, thewashcoat is applied to a substrate in a loading of about 0.8 to 1.0g/in³, 1.0 to 1.5 g/in³, 1.5 to 2.5 g/in³, or 2.5 to 3.5 g/in³.

Preferred substrates, particularly for mobile applications, includeflow-through monoliths having a so-called honeycomb geometry thatcomprise multiple adjacent, parallel channels that are open on both endsand generally extend from the inlet face to the outlet face of thesubstrate and result in a high-surface area-to-volume ratio. For certainapplications, the honeycomb flow-through monolith preferably has a highcell density, for example about 600 to 800 cells per square inch, and/oran average internal wall thickness of about 0.18-0.35 mm, preferablyabout 0.20-0.25 mm. For certain other applications, the honeycombflow-through monolith preferably has a low cell density of about 150-600cells per square inch, more preferably about 200-400 cells per squareinch. Preferably, the honeycomb monoliths are porous. In addition tocordierite, silicon carbide, silicon nitride, ceramic, and metal, othermaterials that can be used for the substrate include aluminum nitride,silicon nitride, aluminum titanate, α-alumina, mullite, e.g., acicularmullite, pollucite, a thermet such as Al₂OsZFe, Al₂O₃/Ni or B₄CZFe, orcomposites comprising segments of any two or more thereof. Preferredmaterials include cordierite, silicon carbide, and alumina titanate.

In certain embodiments, the invention is a catalyst article made by aprocess described herein. In a particular embodiment, the catalystarticle is produced by a process that includes the steps of applying thefirst NH₃-SCR catalyst composition, preferably as a washcoat, to asubstrate as a layer either before or after the second NH₃-SCR catalystcomposition, preferably as a washcoat, has been applied to thesubstrate.

In certain embodiments, the second NH₃-SCR catalyst composition isdisposed on the substrate as a top layer and another composition, suchas an oxidation catalyst, reduction catalyst, scavenging component, orNO_(x) storage component, is disposed on the substrate as a bottomlayer.

In general, the production of an extruded solid body containing thefirst or second NH₃-SCR catalyst composition involves blending themolecular sieve and the copper (either separately or together as ametal-exchanged molecular sieve), a binder, an optional organicviscosity-enhancing compound into an homogeneous paste which is thenadded to a binder/matrix component or a precursor thereof and optionallyone or more of stabilized ceria, and inorganic fibers. The blend iscompacted in a mixing or kneading apparatus or an extruder. The mixtureshave organic additives such as binders, pore formers, plasticizers,surfactants, lubricants, dispersants as processing aids to enhancewetting and therefore produce a uniform batch. The resulting plasticmaterial is then molded, in particular using an extrusion press or anextruder including an extrusion die, and the resulting moldings aredried and calcined. The organic additives are “burnt out” duringcalcinations of the extruded solid body. A metal-promoted zeolitecatalyst may also be washcoated or otherwise applied to the extrudedsolid body as one or more sub-layers that reside on the surface orpenetrate wholly or partly into the extruded solid body. Alternatively,a metal-promoted zeolite can be added to the paste prior to extrusion.Preferably, the copper-loaded molecular sieve is dispersed throughout,and preferably evenly throughout, the entire extruded catalyst body.

Extruded solid bodies containing metal-promoted zeolites according tothe present invention generally comprise a unitary structure in the formof a honeycomb having uniform-sized and parallel channels extending froma first end to a second end thereof. Channel walls defining the channelsare porous. Typically, an external “skin” surrounds a plurality of thechannels of the extruded solid body. The extruded solid body can beformed from any desired cross section, such as circular, square or oval.Individual channels in the plurality of channels can be square,triangular, hexagonal, circular etc.

The catalytic article described herein can promote the reaction of anitrogenous reductant, preferably ammonia, with nitrogen oxides toselectively form elemental nitrogen (N₂) and water (H₂O). Examples ofsuch nitrogenous reductants include ammonia and ammonia hydrazine or anysuitable ammonia precursor, such as urea ((NH₂)₂CO), ammonium carbonate,ammonium carbamate, ammonium hydrogen carbonate or ammonium formate. TheSCR process of the present method can result in a NO_(x) (NO and/or NO₂)conversion of at least 75%, preferably at least 80%, and more preferablyat least 90% over a broad temperature range (e.g., about 150-700° C.,about 200-350° C., about 350-550° C., or about 450-550° C.).

Importantly, the use of zoned catalysts according to the presentinvention generates low amounts of N₂O byproduct compared toconventional SCR catalysts. That is, the SCR process of the presentmethod can result in low N₂O generation based on NO and/or NO₂ at theSCR inlet. For example, the relative ratio of inlet NO concentration atthe SCR catalyst compared to outlet N₂O concentration after the SCRcatalyst is greater than about 25, greater than about 30 (for exampleabout 30 to about 40), greater than about 50, greater than about 80, orgreater than about 100 over a broad temperature range (e.g., about150-700° C., about 200-350° C., about 350-550° C., or about 450-550°C.). In another example, the relative ratio of inlet NO₂ concentrationat the SCR catalyst compared to outlet N₂O concentration after the SCRcatalyst is greater than about 50, greater than about 80, or greaterthan about 100 over a broad temperature range (e.g., about 150-700° C.,about 200-350° C., about 350-550° C., or about 450-550° C.).

The copper-loaded molecular sieve catalyst described herein coupled withan oxidation catalyst can also promote the oxidation of ammonia andlimit the undesirable formation of NO_(x) by the oxidation process(i.e., an ammonia slip catalyst (ASC)). In certain embodiments, thecatalytic article of the present invention contains an ASC zone at theoutlet end of the substrate. In other embodiments, an ammonia slipcatalyst disposed on a separate brick downstream of the zoned SCRcatalysts. These separate bricks can be adjacent to, and in contactwith, each other or separated by a specific distance, provided that theyare in fluid communication with each other and provided that the SCRcatalyst brick is disposed upstream of the ammonia slip catalyst brick.

In certain embodiments, the SCR and/or ASC process is performed at atemperature of at least 100° C. In another embodiment, the process(es)occur at a temperature from about 150° C. to about 750° C. In aparticular embodiment, the temperature range is from about 175 to about550° C. In another embodiment, the temperature range is from 175 to 400°C. In yet another embodiment, the temperature range is 450 to 900° C.,preferably 500 to 750° C., 500 to 650° C., 450 to 550° C., or 650 to850° C.

According to another aspect of the invention, provided is a method forthe reduction of NO_(X) compounds and/or oxidation of NH₃ in a gas,which comprises contacting the gas with a catalyst described herein fora time sufficient to reduce the level of NO_(x) compounds in the gas.Methods of the present invention may comprise one or more of thefollowing steps: (a) accumulating and/or combusting soot that is incontact with the inlet of a filter; (b) introducing a nitrogenousreducing agent into the exhaust gas stream prior to contacting the SCRcatalyst, preferably with no intervening catalytic steps involving thetreatment of NO_(x) and the reductant; (c) generating NH₃ over a NO_(x)adsorber catalyst or lean NO_(x) trap, and preferably using such NH₃ asa reductant in a downstream SCR reaction; (d) contacting the exhaust gasstream with a DOC to oxidize hydrocarbon based soluble organic fraction(SOF) and/or carbon monoxide into CO₂, and/or oxidize NO into NO₂, whichin turn, may be used to oxidize particulate matter in particulatefilter; and/or reduce the particulate matter (PM) in the exhaust gas;(e) contacting the exhaust gas with one or more downstream SCR catalystdevice(s) (filter or flow-through substrate) in the presence of areducing agent to reduce the NOx concentration in the exhaust gas; and(f) contacting the exhaust gas with an ammonia slip catalyst, preferablydownstream of the SCR catalyst to oxidize most, if not all, of theammonia prior to emitting the exhaust gas into the atmosphere or passingthe exhaust gas through a recirculation loop prior to exhaust gasentering/re-entering the engine.

In a preferred embodiment, all or at least a portion of thenitrogen-based reductant, particularly NH₃, for consumption in the SCRprocess can be supplied by a NO_(x) adsorber catalyst (NAC), a leanNO_(x) trap (LNT), or a NO_(x) storage/reduction catalyst (NSRC),(collectively NAC) disposed upstream of the SCR catalyst. In certainembodiments, the NAC is coated on the same flow-through substrate as thezoned SCR catalyst. In such embodiments, the NAC and SCR catalysts arecoated in series with the NAC being upstream of the SCR zones.

NAC components useful in the present invention include a catalystcombination of a basic material (such as alkali metal, alkaline earthmetal or a rare earth metal, including oxides of alkali metals, oxidesof alkaline earth metals, and combinations thereof), and a preciousmetal (such as platinum), and optionally a reduction catalyst component,such as rhodium. Specific types of basic material useful in the NACinclude cesium oxide, potassium oxide, magnesium oxide, sodium oxide,calcium oxide, strontium oxide, barium oxide, and combinations thereof.The precious metal is preferably present at about 10 to about 200 g/ft³,such as 20 to 60 g/ft³. Alternatively, the precious metal of thecatalyst is characterized by the average concentration which may be fromabout 40 to about 100 grams/ft³.

Under certain conditions, during the periodically rich regenerationevents, NH₃ may be generated over a NO_(x) adsorber catalyst. The SCRcatalyst downstream of the NO_(x) adsorber catalyst may improve theoverall system NO_(x) reduction efficiency. In the combined system, theSCR catalyst is capable of storing the released NH₃ from the NACcatalyst during rich regeneration events and utilizes the stored NH₃ toselectively reduce some or all of the NO_(x) that slips through the NACcatalyst during the normal lean operation conditions.

In certain aspects, the invention is a system for treating exhaust gasgenerated by combustion process, such as from an internal combustionengine (whether mobile or stationary), a gas turbine, coal or oil firedpower plants, and the like. Such systems include a zoned SCR catalyticarticle described herein and at least one additional component fortreating the exhaust gas, wherein the zoned SCR catalytic article and atleast one additional component are designed to function as a coherentunit. The zoned SCR catalytic article and at least one additionalcomponent are in fluid communication, optionally by one or more sectionsof conduit for channeling exhaust gas through the system.

The exhaust gas treatment system can comprise an oxidation catalyst(e.g., a diesel oxidation catalyst (DOC)) for oxidizing nitrogenmonoxide in the exhaust gas to nitrogen dioxide can be located upstreamof a point of metering the nitrogenous reductant into the exhaust gas.In one embodiment, the oxidation catalyst is adapted to yield a gasstream entering the SCR zeolite catalyst having a ratio of NO to NO₂ offrom about 4:1 to about 1:3 by volume, e.g. at an exhaust gastemperature at oxidation catalyst inlet of 250° C. to 450° C. Theoxidation catalyst can include at least one platinum group metal (orsome combination of these), such as platinum, palladium, or rhodium,coated on a flow-through monolith substrate. In one embodiment, the atleast one platinum group metal is platinum, palladium or a combinationof both platinum and palladium. The platinum group metal can besupported on a high surface area washcoat component such as alumina, azeolite such as an aluminosilicate zeolite, silica, non-zeolite silicaalumina, ceria, zirconia, titania or a mixed or composite oxidecontaining both ceria and zirconia.

The exhaust gas treatment system can comprise an additional SCR catalyston a second flow-through monolith or a wall-flow filter, wherein thesecond flow-through monolith or wall-flow filter containing theadditional SCR is positioned upstream or downstream of, and in fluidcommunication with, the zoned SCR catalytic article described herein.The additional SCR catalyst is preferably a metal-exchanged zeolite,such as Cu-Beta, Cu-ZSM5, Cu-CHA, Cu-ZSM-34, or Cu-AEI.

The exhaust gas treatment system can comprise an NAC and/or an externalsource of nitrogenous reductant (e.g., an ammonia or urea injector)disposed upstream of the catalytic article. The system can include acontroller for the metering the external nitrogenous reductant into theflowing exhaust gas only when it is determined that the SCR catalystzones are capable of catalyzing NO_(x) reduction at or above a desiredefficiency, such as at above 100° C., above 150° C. or above 175° C. Themetering of the nitrogenous reductant can be arranged such that 60% to200% of theoretical ammonia is present in exhaust gas entering the SCRcatalyst calculated at 1:1 NH₃/NO and 4:3 NH₃/NO₂.

The exhaust gas treatment system can comprise a suitable particulatefilter, such as a wall-flow filter. Suitable filters include thoseuseful in removing soot from an exhaust gas stream. The filter can bebare and passively regenerated, or can contain a soot combustioncatalyst or a hydrolysis catalyst. The filter can also comprise an SCRcatalyst loaded on the inlet side of the filter walls, the outlet sideof the filter walls, partially or fully permeate the filter walls, orsome combination thereof. In certain embodiments, the filter is thesubstrate for the first or second catalyst zone as described herein,provided that the alternate zone is disposed on a flow-throughsubstrate. For example, a wall flow filter can be used as a substratefor the first zone and a flow-through honeycomb can be used as thesubstrate for the second zone. In another example, a flow-throughhoneycomb can be used as the substrate for the first zone and awall-flow filter can be used as the substrate for the second zone. Insuch embodiments, the wall flow substrate may further comprise an NH₃oxidation catalyst to form an ASC zone.

The filter can be positioned in the exhaust gas treatment system eitherupstream or downstream of the zoned SCR catalyst. Preferably, the filteris positioned downstream of the DOC if a DOC is present. For embodimentscomprising a bare filter (i.e. having no catalyst coating) and anammonia injector upstream of the zoned SCR catalyst, the injector can bepositioned upstream or downstream of the filter provided that it ispositioned upstream of the zoned SCR catalyst. For embodiments having afilter containing a hydrolysis catalyst and downstream zoned SCRcatalyst, an ammonia injector is preferable positioned upstream of thefilter.

Turning to FIG. 11, shown is an exhaust gas treatment system comprisingan internal combustion engine 501, an exhaust gas treatment system 502,a direction of exhaust gas flow through the system 1, an optional DOC510 and/or an optional NAC 520, an optional particulate filter 570, anoptional external source of ammonia and injector 530, a zoned SCRcatalyst 540, an optional additional SCR catalyst 550, and an optionalASC 560.

The method for treating exhaust gas as described herein can be performedon an exhaust gas derived from a combustion process, such as from aninternal combustion engine (whether mobile or stationary), a gas turbineand coal or oil fired power plants. The method may also be used to treatgas from industrial processes such as refining, from refinery heatersand boilers, furnaces, the chemical processing industry, coke ovens,municipal waste plants and incinerators, etc. In a particularembodiment, the method is used for treating exhaust gas from a vehicularlean burn internal combustion engine, such as a diesel engine, alean-burn gasoline engine or an engine powered by liquid petroleum gasor natural gas.

What is claimed is:
 1. A catalytic article comprising: a. a flow throughhoneycomb substrate having an inlet side, an outlet side, an axiallength from the inlet side to the outlet side, and a plurality ofchannels defined by channel walls extending from the inlet side to theoutlet side; b. a first NH₃-SCR catalyst composition coated on and/orwithin the channel walls in a first zone; and c. a second NH₃-SCRcatalyst composition coated on and/or within the channel walls in asecond zone, provided that the first zone is upstream of the second zoneand the first and second zones are adjacent or at least partiallyoverlap; wherein the first NH₃-SCR catalyst comprises a first copperloaded molecular sieve having a copper to aluminum atomic ratio of about0.1 to 0.375 and the second NH₃-SCR catalyst comprises a second copperloaded molecular sieve having a copper-to-aluminum atomic ratio of about0.3 to about 0.6.
 2. The catalytic article of claim 1, wherein the firstzone is adjacent to the second zone.
 3. The catalytic article of claim1, wherein the first zone completely overlays the second zone.
 4. Thecatalytic article of claim 1, wherein first zone extends from the inletside to a first end point that is position about 10 to 40 percent of theaxial length and wherein the second zone is about 20 to 90 percent ofthe axial length, provided that the first and second zones are adjacentor overlap by less than 90 percent of the axial length.
 5. The catalyticarticle of claim 4, wherein the first zone overlaps the second zone. 6.The catalytic article of claim 4, wherein the second zone overlaps thefirst zone.
 7. The catalytic article of claim 1, wherein the firstcopper loaded molecular sieve has a lower copper concentration relativeto the second copper loaded molecular sieve.
 8. The catalyst article ofclaim 2, wherein the first copper loaded molecular sieve has a copperloading of about 50 to 90 percent of second copper loaded molecularsieve.
 9. The catalytic article of claim 1, wherein the first copperloaded molecular sieve has a first SAR and the second copper loadedmolecular sieve has a second SAR which is larger than the first SAR. 10.The catalyst article of claim 9, wherein the first molecular sieve is analuminosilicate having a SAR of about 10 to about 20 and the secondmolecular sieve is an aluminosilicate having a SAR of about 20 to about50.
 11. The catalyst article of claim 9, wherein the first and secondmolecular sieves have a small pore framework.
 12. The catalyst articleof claim 9, wherein the first and second molecular sieves have a CHAframework.
 13. The catalyst article of claim 9, wherein the firstmolecular sieve is a silicoaluminophosphate and the second molecularsieve is an aluminosilicate having a SAR of about 15 to about
 50. 14.The catalyst article of claim 4, wherein the first and second zonesoverlap to create a third catalyst zone, wherein the third zone having ahigher amount of copper compared to the first and second zones,individually.
 15. The catalyst article of claim 1, further comprising anASC zone having an ammonia oxidation catalyst coated on and/or withinthe channel walls, wherein the ASC zone extends from the outlet side fora distance that is about 10 to 50 percent of the axial length and doesnot contact the first zone.
 16. The catalyst article of claim 15,wherein the second NH₃-SCR catalyst composition fully overlaps theoxidation catalyst.
 17. The catalyst article of claim 16, wherein theoxidation catalyst comprises platinum.
 18. A catalyst articlecomprising: a. a flow through honeycomb substrate having an inlet side,an outlet side, an axial length from the inlet side to the outlet side,and a plurality of channels defined by channel walls extending from theinlet side to the outlet side, b. a first catalytic zone consisting of afirst washcoat, c. a second catalytic zone consisting of the firstwashcoat and a second washcoat, d. a third catalytic zone consisting ofthe second washcoat, and e. a fourth catalytic zone consisting of thesecond washcoat over a third washcoat, wherein the first washcoatcontains a first copper loaded molecular sieve, the second washcoatcontains a second copper loaded molecular sieve wherein the first andsecond molecular sieves are different materials, and the third washcoatcontains an ammonia oxidation catalyst, and wherein the first, second,third, and fourth zones are arranged in series on the substrate, eachzone is adjacent to the next zone in the series, the first zone isproximal to the inlet side, and the fourth zone is proximal to theoutlet side.
 19. A system for treating an exhaust gas comprising: a. acatalytic article according to claim 1; and b. one or more exhaust gastreatment components selected from DOC, NAC, external NH₃ injector,secondary SCR catalyst, ASC, and particulate filter, wherein thecatalytic article according to claim 1 and the one or more exhaust gastreatment components are in fluid communication and are in series.
 20. Amethod for treating an exhaust gas comprising: contacting an exhaust gascomprising NO_(x) and NH₃ with a catalytic article according to claim 1;selectively reducing at least a portion of the NO_(x) to N₂ and H₂O.